Novel discrete frequency-phase modulated excitation waveform for enhanced depth resolvability of thermal wave radar

Abstract

Thermal wave radar (TWR) is a state-of-the-art non-destructive testing method, inspired by radio wave radar systems, in order to increase depth resolution and signal to noise ratio of optical infrared thermography through pulse compression. Analogue frequency modulation (i.e. frequency sweep) and Barker binary phase modulation are the two popular and widely researched pulse compression techniques in TWR among which Barker coding has shown the highest performance. This paper introduces a novel modulated waveform with variable discrete frequencyphase modulation (FPM) which distinctively enhances the depth resolvability of TWR compared to the existing techniques. The pulse compression quality and depth resolvability of the novel FPM waveform is initially evaluated through a 1D analytical solution. The analogue frequency modulated and discrete phase modulated waveforms as well as mono-frequency excitation (i.e. lock-in thermography) are also evaluated at the same central frequency as the reference. Objective functions are defined and a large search space is explored for optimal modulation codes. Two FPM waveforms are selected based on their maximized depth resolvability through resultant lag and phase in the output channel of TWR. Furthermore, the excellent performance of the selected FPM waveforms is validated by 3D finite element simulation. A delaminated glass fiber reinforced polymer (GFRP) laminate is simulated in order to evaluate the impact of a dominant lateral heat diffusion on the performance of the novel FPM waveforms. The superior depth resolvability of the introduced FPM waveforms is confirmed and their robustness at various noise levels is demonstrated.

Vibro-Thermal Wave Radar: Application of Barker Coded Amplitude Modulation for Enhanced Low-Power Vibrothermographic Inspection of Composites

“…Recently, the current authors introduced a discrete frequency-phase modulated waveform which was the outcome of an optimization study. This novel frequency-phase modulated waveform outperforms the existing waveforms in terms of depth resolvability [ 34 , 35 , 36 ]. The TWR approach is not exclusive to the case of optical heating, and it has already been applied for enhanced performance of eddy current infrared thermography [ 37 , 38 , 39 ].…”

“…Analogue frequency modulated (sweep) and discrete phase modulated (Barker coded) excitations are the two widely researched types of modulated waveforms in TWR [20,[24][25][26][27][28]. Recently, the current authors introduced a novel optimized discrete frequency-phase modulated waveform which outperforms the existing waveforms in terms of depth resolvability [29,30]. The TWR approach is not exclusive to optical heating, and it has already been applied for enhanced performance and SNR of eddy current infrared thermography [31][32][33][34].…”

Phase inversion in (vibro‐)thermal wave imaging of materials: Extracting the AC component and filtering nonlinearity

“…• a cyclic sinusoidal (i.e., lock-in) 13,[35][36][37][38][39] or stepwise 6 excitation at a fixed modulation frequency may be applied to increase the SNR, or • a frequency-modulated (e.g., sweep) excitation signal may be applied to tune the depth range of the inspection, 40,41 or • a frequency-and/or phase-modulated excitation signal may be applied to reconstruct an impulse response of the material through the pulse compression technique (i.e., thermal wave radar). [42][43][44][45][46] All these excitation signals have a bipolar definition, meaning that their application in thermography would require active heating and cooling of the inspection surface. This is in contrast to the monopolar nature of commonly used excitation sources which can only introduce heat to the sample.…”

Multi-scale gapped smoothing algorithm for robust baseline-free damage detection in optical infrared thermography

“…In lock-in thermography (LT), halogen lamps are employed to excite the sample with a single-frequency sinusoidal signal [2][3][4], while in phase-locked restored pseudo heat flux thermography the halogen lamps excite with a periodic square wave [5]. Thermal wave radar (TWR) on the other hand exploits the use of advanced frequency and/or phase modulated waveforms for better defect detectability and depth probing [6][7][8][9]. Stepheating thermography (SHT) [10] and the recently proposed long-pulse thermography (LPT) [11,12] both apply step-pulse heating for a specific period of time (e.g.…”